The present invention relates to spline connections, and more particularly, to spline connections in which one of the sets of splines comprises crowned, involute external splines.
It should become apparent from the subsequent detailed description that the present invention is applicable wherever a shaft is provided with a set of crowned, involute external splines which transmit torque to or from a set of straight, involute internal splines, wherein the internal and external splines cooperate to act as a form of universal connection, wherein the axis of the shaft having the external splines is not parallel to the axis of the element associated with the set of internal splines. Therefore, it should be understood that the present invention may be applied to a single set of crowned, external splines on a shaft having a different form of universal connection at its other end, as well as to a drive shaft having sets of crowned, external splines at both ends.
The invention is especially advantageous when used in a drive connection for transmitting torque between a member having pure rotational motion and a member having both orbital and rotational motion. Such a drive arrangement is commonly found in rotary fluid pressure devices (either a motor or pump) which include a gerotor gear set, and the present invention will be described in connection therewith.
Fluid motors of the type utilizing a gerotor gear set to convert fluid pressure into a rotary output have become popular and are especially suited for low speed, high torque applications. In most of the commercially available fluid motors of this type, one of the primary factors limiting the torque output capability of the motor is the strength of the drive connection which transmits torque from the orbiting and rotating member (rotor) of the gerotor gear set to the output shaft of the motor. Typically, this drive connection comprises a set of internal splines defined by the rotor, a set of internal splines defined by an enlarged portion of the output shaft and a main drive shaft having a set of external splines at each end thereof in engagement with the sets of internal splines. Generally, the internal splines are straight whereas the external splines are crowned to take into account the angle at which the drive shaft is oriented relative to the axis of rotation of the motor.
Crowned splines are usually manufactured by a process referred to as "rise-and-fall" hobbing, especially when a large volume of production is involved. As is well known in the art, when a set of crowned external splines is produced by rise-and-fall hobbing, the individual splines are normally symmetrical about a central plane oriented perpendicular to the axis of the drive shaft, the plane intersecting each of the splines at the theoretical "peak" of the crown. For purposes of simplicity in describing the present invention, it will be assumed that the `crown angle` and the `rise and fall` angle are equal, and may be used interchangeably. As is also well known in the art, the crown angle of the external splines should be sufficient that at any axial distance from the central plane along the external spline, there will be a clearance between the external spline and the adjacent internal spline (measured perpendicular to the flank of the external spline with the axis of the shaft parallel to the axis of the internal splines) to prevent interference between adjacent splines during operation of the device.
Among the factors which determine the torque carrying capacity of this type of drive connection are the spline length (i.e., the axial length of the individual external splines), and the shank diameter (i.e., the diameter of the shaft between the sets of external splines). As has been recognized by those skilled in the art, an attempt to increase the torque carrying capacity of a drive shaft by increasing either tooth length or shank diameter is limited by the interdependence of the two factors. On external splines cut by rise-and-fall hobbing, as the tooth length is increased, the depth of cut toward the ends of the spline set increases. Thus, the shank diameter would actually have to be decreased; otherwise, the hob cutter would cut into the shank adjacent the end of the spline, weakening the shank. Conversely, if the shank diameter is increased, it would be necessary to decrease the spline length to reduce the depth of cut at the ends of the spline, again to prevent the hob cutter from cutting into the shank.
Although the background of the invention has been discussed in terms of torque carrying capacity, it is also related to spline life. As a rough guideline, it is generally recognized that spline life is approximately proportional to spline length for a given torque, or that torque is approximately proportional to spline length for a given spline life.